Reference Infrastructure for Altimetry Calibration

Permanent Facility for Altimetry Calibration (PFAC)

The European Space Agency (ESA) Permanent Facility for Altimetry Calibration (ESA-PFAC) is an operational ground reference infrastructure that has been providing multi-mission, multi-technique Cal/Val services for about 20 years. The PFAC is distributed over Crete and Gavdos islands. It consists of several permanent and independent Cal/Val reference sites which are discriminated in five main categories based on the calibration employed and their contribution to the Cal/Val altimeter services supported by the ESA-PFAC.

Transponder Cal/Val Facilities;

Coastal Infrastructure for Sea-surface Cal/Val ;

Corner Reflectors Infrastructure;

GNSS Interferometric Reflectometry;

Sea-State Optical Monitoring Framework ;

Transponder Cal/Val Facilities

Microwave transponders are used as active ground point targets of reference. They receive, amplify, and send back to the satellite the radar signal emitted by the satellite altimeter. Using transponders, the distance between the satellite altimeter and the transponder as well as the time-tagging bias can be precisely and directly measured. Also, the back-scattering coefficient can be calibrated.

In the ESA-PFAC, two transponder Cal/Val sites at locations in Crete (CDN1) and Gavdos (GVD1) continuously support the calibration of range (CDN1, GVD1), time-tagging, and backscatter coefficient (GVD1) of satellite altimetry observations.

Crete: CDN1 Cal/Val Transponder Site

The CDN1 transponder Cal/Val site is located on a mountainous area, at 1,000 m altitude in the mainland of West Crete. It has been established primarily to support Cal/Val activities for the Sentinel- 3A & Sentinel-3B. It also serves Sentinel-6 MF, CryoSat-2 and SWOT satellite altimeters.

The scientific and ancillary instrumentation operating at the CDN1 transponder Cal/Val site are:

Cal/Val site Instrumentation:
- Ku-band range transponder,
- Two GNSS receivers,
- Two meteorological stations,
- Dedicated Internet Access,
- Satellite link,
- Microwave radiometer (decommissioned),
- Power supply systems (solar, wind, batteries, diesel generator).
Instrument Operating Procedures
- GNSS & Meteo: 24/7/365,
- Radiometer: 24/7/365,
- Transponder: -1 h to + 10 min after satellite overpass.

On 2 October 2015, the first transponder calibration was completed at the CDN1 Cal/Val site in Crete. Since then, more than 600 multi-mission calibrations have been achieved with 92% success rate .

The performance of the CDN1 transponder Cal/Val

The two transponders in Crete (a) and Gavdos(b) and their GNSS infrastructure.

Gavdos: GVD1 Transponder Cal/Val Site

The GVD1 transponder Cal/Val site is located in Gavdos under a crossover of ascending and descending orbits of Sentinel-6 MF. It also supports Sentinel-3A, CryoSat-2, while it may be used for calibrating the SWOT nadir altimeter.

The scientific and ancillary instrumentation operating at the GDV1 transponder Cal/Val site are:

Cal/Val Site Instrumentation:
- Range & sigma-0 transponder in Ku-band,
- Three GNSS receivers,
- One DORIS beacon,
- Three meteorological stations,
- Dedicated Internet Access,
- Satellite link,
- Power supply systems (mains, batteries, diesel generator)
Instrument Operating Procedures
- GNSS & Meteo: 24/7/365,
- DORIS: 24/7/365,
- Transponder: -1 h to + 10 min after satellite overpass

This GVD1 transponder supports the goals of the FRM strategic plan for monitoring satellite altimetry. It provides redundancy in monitoring Sentinel-6 MF as the same pass is evaluated using both CDN1 and GVD1 transponders. The same is true for Sentinel-3A, CryoSat-2 and SWOT satellites while orientation errors (ascending vs descending orbits) are to monitored by this pair of active transponders.

For the first time, simultaneous range and sigma-0 (backscatter coefficient) calibration is carried out at the GVD1 transponder Cal/Val.

Since 11-Oct-2021, more than 250 multi-mission calibration were scheduled at the GVD1 Cal/Val with a success rate of more than 96%.

The performance of the GVD1 transponder Cal/Val

Coastal Infrastructure for Sea-surface Cal/Val

Four (CRS1, SUG1, GVD8, RDK1) permanent sea-surface calibration facilities are part of the PFAC infrastructure while two more (KML1, KRM1) are operational ready. These are all equipped with diverse tide gauges, GNSS receivers, meteorological sensors, etc. and provide the ground truth measurements of sea-surface height. The bias of the sea-surface height as measured by the satellite altimeter is thus determined.

Site	Coordinates	Missions	Scientific Instrumentation
Gavdos (GVD8)	Lat:34.8479730 N Lon:24.1197700 E	S6, S3A, CryoSat-2, Jason-3	5 Tide gauges 3 GNSS stations 1 Meteo station
RDK1	Lat:35.1875970 N Lon:24.3184830 E	S6, S3A, S3B, HY-2B, Jason-3	2 Tide gauges 1 GNSS station 1 Meteo station
CRS1	Lat:35.3031510 N Lon:24.5213510 E	S3A, S3B, HY-2B	2 Tide gauges 1 GNSS station 1 Meteo station
SUG1	Lat:35.2457950 N Lon:23.8049270 E	S6, S3A, S3B, CryoSat-2, Jason-3	2 Tide gauges 1 GNSS station 1 Meteo station

The four operational sea-surface Cal/Val sites: Gavdos (up left), CRS1 (up right), RDK1 (down left) and SUG1 (down right).

Corner reflector Cal/Val Infrastructure

Corner reflectors constitutes a standard procedure in radar imaging. In satellite altimetry, corner reflectors have recently gained popularity for range calibration in the Fully-Focused technique.

Two corner reflectors have been designed, and manufactured by the PFAC team for the calibration of Ku- and Ka-band satellite altimeters. On 29 July 2023, the first corner reflector was installed at the ALX1 Cal/Val site, and a few months later on 20 January 2024, the ALX2 completed the installation in Crete.

GNSS Interferometric Reflectometry

The GNSS-Interferometric Reflectometry (GNSS-IR) technique is an alternative to conventional sea-surface Cal/Val particularly for significant wave height and wind calibration. It uses the signal-to-noise ratio data of GNSS observations to parameters such as water level and wave height.

A GNSS-IR experiment was conducted in Gavdos in the period 30-Nov-2023 to 7-Dec-2023. The purpose of this experiment was to examine if we can observe sea state (i.e., significant wave height, SWH) with the GNSS-IR technique. Two temporary locations in Gavdos were identified as candidate sites for this GNSS-IR experiment.

The results of this experiment were quite satisfactory and the installation of permanent GNSS stations in Gavdos is currently under investigation.

Sea-State Optical Infrastructure

The main purpose of the Sea State Optical (SSO) technique is to assess sea state variability and sub-mesoscale surface processes and evaluate their impact on satellite altimeter measurements.

The sea state variability can be studied either by measuring the Mean Square Slope from the Sun glitter intensity or to reconstruct the ocean surface geometry using stereo photo reconstruction technique. The sub-mesoscale processes can be measured by analyzing the wave dispersion relation estimated from video recording of the sun glitter.

Therefore, we have designed and procured an optical imaging system, comprised by two cameras, to study both the sea state variability and the sub-mesoscale processes. A thorough study on the specifications that these cameras should meet has been carried out along with the selection of the optimal location from their installation. A location at the mountains of south Crete has been selected so that it could be used for both ascending and descending passes of Sentinel-6 MF.

On 19 December 2023, the installation of sea-state optical imaging system was completed at two locations identified as SFS1-East and SFS-1 West in Crete overlooking Gavdos.

The two cameras installed west (left) and east (right) at the SFS1 location in south Crete, Greece.

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